The electrospray process consists of flowing a sample liquid through a small tube or needle, which is maintained at a high voltage relative to a nearby surface. The voltage gradient at the tip of the needle causes the liquid to be dispersed into fine electrically charged droplets. The ionization mechanism involves desorption at atmospheric pressure of ions from the fine electrically charged particles. In many cases a heated gas is flowed in a direction that is counter-current to the electrospray, so as to enhance desolvation of the electrosprayed droplets. The ions created by the electrospray process are then mass analyzed using a mass analyzer.
Under appropriate conditions the electrospray resembles a symmetrical cone consisting of a very fine mist of droplets of ca. 1 μm in diameter. Excellent sensitivity and ion current stability is obtained if a fine mist is produced. Unfortunately, the electrospray “quality” is highly dependent on the bulk properties of the solution that is being analyzed, such as for instance surface tension and conductivity. A poor quality electrospray contains larger droplets of greater than 10 μm diameter, or a non-dispersed droplet stream.
The use of a sheath liquid and a focusing gas helps to ensure stable sprays when electrospraying high aqueous content sample solutions. One type of electrospray interface includes an inner needle for transferring a liquid sample to an ionizing region at one end of the needle, a first outer tube surrounding and spaced from said needle for flowing a sheath liquid past the tip of said needle, and a second outer tube surrounding the first tube to define a second cylindrical space for flowing a focusing gas past the end of said first tube and needle to focus the electrospray.
In U.S. Pat. No. 4,542,293, the entire contents of which is incorporated herein by reference, there is described the use of a tube made of an electrical insulator for conducting ions between the ionizing electrospray region at atmospheric pressure and an adjacent low-pressure region. A glass or quartz capillary is suitable for this purpose. Ions and gas are caused to flow from the ionization region through the tube and into the low-pressure region where free jet expansion occurs. A conductive coating is formed on the ends of the insulating tube and a voltage is applied thereacross to accelerate ions as they flow through the tube. A conducting skimmer is disposed adjacent the end of the tube and is maintained at a voltage which causes further acceleration of the ions through and into a lower pressure region including ion focusing lenses and analyzing apparatus.
In U.S. Pat. No. 5,171,990, the entire contents of which is incorporated herein by reference, there is described an electrospray ion source of the type which includes a capillary tube communicating between the ionizing region and a low-pressure region with a skimmer having an aperture through which ions pass. The skimmer separates the low-pressure region from a progressively lower pressure region, which includes ion focusing lenses and an analyzer. The capillary tube is oriented so that undesolvated droplets or particles travelling through the capillary are prevented from passing through the skimmer aperture into the analysis region. In particular, the axis of the capillary is altered or directed so that the axis is offset from the skimmer orifice. In this way, there is no alignment between the bore of the capillary and the orifice of the skimmer. The tendency is for the large droplets or particles to move to the center of the flow in the capillary and travel in a straight line. These droplets or particles traveling in a straight line strike the skimmer. The droplets or particles are thereafter pumped away. Unfortunately, the off-axis arrangement results in a portion of the electrosprayed sample being “clipped” such that the ion signal is reduced. Furthermore, the sample matrix tends to build up over time on the surface of the skimmer, which necessitates periodic cleaning and maintenance.